1. Field of the Invention
The present invention relates to an electrochromic element and methods of forming an electrochromic element capable of controlling transmittance of light.
2. Description of the Related Art
Electrochromism is a phenomenon shown by materials in which a reversible electrochemical reaction is induced in response to a supply of voltage, which changes the absorption wavelength range of the material for visible light, as a result of which the material develops or loses a color. An electrochemically colored or bleached element using such an electrochromic phenomenon is referred to as an “electrochromic element”. An electrochromic element can be used as a light adjustment (or light modulating) element for controlling transmittance of visible light. As used herein, the term “transmittance” is to be accorded its ordinary meaning, which can be loosely defined as the ratio of the light energy falling on a body to that transmitted through it. The term “visible light” refers to electromagnetic radiation in wavelengths ranging substantially within 350 and 850 nanometers (nm) inclusive.
FIG. 2 illustrates an example of the structure of an electrochromic element. As illustrated in FIG. 2, an electrochromic element 200 is constructed by sequentially stacking a transparent first substrate 1, a transparent first electrode 2, an electrochromic layer 3, an electrolyte layer 4, a transparent second electrode 5, and a transparent second substrate 6.
It is useful to configure a transmissive electrochromic element to suppress reflection of visible light on a boundary surface between layers included in the element. When visible light is reflected on a boundary surface between layers included in an element, the transmittance of the visible light transmitted through the element is low. Further, the light reflected from the boundary surface interferes with incident light, thereby causing a phenomenon of low transmittance of the visible light of a specific wavelength, i.e., wavelength dependence of visible light transmittance.
Reflectance R of light on a boundary surface between adjacent layers respectively having refractive indices n1 and n2 is given by Fresnel's formula expressed by:R=(n1−n2)2/(n1+n2)2  (EQUATION 1).From EQUATION 1, it can be appreciated that as the difference between n1 and n2 increases, the reflectance increases. Accordingly, transmittance can be controlled by specifically designing refractive indices of adjacent layers in a given optical element.
Japanese Patent Application Laid-Open No. 54-33745 discusses reducing the reflectance on the boundary surface between the layers by selecting the materials of the adjacent layers such that an electrolyte layer and an electrode adjacent to an electrochromic layer have refractive indices close to the refractive index of the electrochromic layer. Since tungsten oxide has a refractive index of approximately 2.0 at a wavelength 550 nm, the adjacent layers also made to have refractive indices of values close to 2.0.
However, when an electrochromic element is prepared by an electrochromic material having a high refractive index, it is difficult to make the refractive index of a layer adjacent to the electrochromic layer from such a material that shows a refractive index close to the refractive index of the electrochromic layer. As a result, the reflectance on the boundary surface is increased, leading to a high possibility of interference of reflected light with indent light.
Titanium oxide is more transparent than tungsten oxide and other inorganic electrochromic materials. Accordingly, titanium oxide is attracting attention as an electrochromic material. However, titanium oxide has a high refractive index of approximately 2.5, and therefore it is not easy to reduce the reflectance only by relying on selection of a material of an adjacent layer of the electrochromic layer.